It is well known that cardiac arrhythmias, such as atrial or ventricular fibrillation, can be overcome by applying electrical energy to the fibrillating myocardium. This procedure, cardioversion or defibrillation, can be accomplished by applying the electrical energy either to the chest of the patient by means of conductive-metal paddles held in place by medical personnel, or during the course of cardiac surgery, by holding conductive-metal paddles in direct contact with the surface of the heart. Such procedures are well known and have been found to be generally effective.
More recently, implantable defibrillators have been proposed for automatically detecting the onset of the cardiac arrhythmia and for automatically correcting such arrhythmia. These automatic defibrillators may employ conformal electrodes, which are maintained in contact with the surface of the heart, or electrodes on an intravascular catheter, or some combination of these. In any case, the electrodes act to impart the desired electrical energy to the heart muscle to achieve difibrillation.
With the intravascular catheter electrode approach, it has been found that, although less electrical energy need be imparted to the heart than in the exterior chest paddles approach, more energy is needed than in the system wherein the electrodes are placed directly in contact with the heart surface. In other words, it has been found that physically placing the electrodes in contact with the exterior of the heart will provide a more efficient use of the electrical energy, thereby reducing the amount of energy required. Obviously, because of a desire to keep the size of the implanted device to a minimum, energy consumption within the device is of the utmost importance.
In the automatic defibrillators previously under consideration, certain of the conformal defibrillation electrodes have been designed for application to the heart by entering the chest cavity and by sewing the electrodes to the heart or positioning the electrodes on the surface of the heart. At times, such electrode implantation may be accomplished during the course of cardiac surgery, such as during a bypass operation. However, even when such heart surgery is not independently required, the earlier surface electrodes required that the chest cavity be opened in order to implant the defibrillating electrodes. This surgical procedure could require intubation of the lungs, and exposes the surfaces of the lungs to possible infection. Additionally, in order for the surgeon to have sufficient working space to effectively position and apply the electrodes, it may be necessary to perform an additional surgical procedure involving spreading two adjacent ribs or splitting the sternum. Accordingly, with these earlier conformal cardiac electrodes, it was necessary to perform major surgery.
Recently, a cardiac electrode has been developed that can be implanted without major surgery. Such an electrode is described in detail in copending U.S. patent application Ser. No. 34,730, entitled "Implantable Cardiac Defibrillating Electrode". Basically, the electrode has a metallic mesh electrode surface surrounded by a protective insulating material. The electrode may be implanted without major surgery by means of an insertion tool which cooperates with a pocket formed in the electrode. After placement of the electrode proximate the heart, the electrode is secured.
Although this new electrode is a significant step forward, there still is room for improvement and a need for a single cardiac defibrillating electrode that can conveniently be positioned and secured proximate hearts of varying sizes without the need for a major surgical procedure. The present invention is directed toward providing that improvement and filling that need.